Ion-Conducting Ceramic Apparatus, Method, Fabrication, and Applications

ABSTRACT

A c-axis-oriented HAP thin film synthesized by seeded growth on a palladium hydrogen membrane substrate. An exemplary synthetic process includes electrochemical seeding on the substrate, and secondary and tertiary hydrothermal treatments under conditions that favor growth along c-axes and a-axes in sequence. By adjusting corresponding synthetic conditions, an HAP this film can be grown to a controllable thickness with a dense coverage on the underlying substrate. The thin films have relatively high proton conductivity under hydrogen atmosphere and high temperature conditions. The c-axis oriented films may be integrated into fuel cells for application in the intermediate temperature range of 200-600° C. The electrochemical-hydrothermal deposition technique may be applied to create other oriented crystal materials having optimized properties, useful for separations and catalysis as well as electronic and electrochemical applications, electrochemical membrane reactors, and in chemical sensors.

RELATED APPLICATION DATA

The instant application claims priority to U.S. Provisional applicationSer. No. 61/031,492 filed on Feb. 26, 2008 and to U.S. Provisionalapplication Ser. No. 61/101,314 filed on Sep. 30, 2008, the subjectmatters of which are hereby incorporated by reference in theirentireties.

FEDERALLY SPONSORED RESEARCH

Certain embodiments and aspects of the disclosed invention were madewith government support under Contract Nos. DE-FG02-05ER15722 andDE-FC03-92SF19460 awarded by the United States Department of Energy. TheUnited States government may have certain rights in the invention.

BACKGROUND

1. Field of the Invention

Embodiments of the invention pertain generally to the field of ionand/or proton conducting membranes. More particularly, embodiments ofthe invention are directed to ion and/or proton conducting membranes,devices incorporating ion and/or proton conducting membranes, methods offabrication of ion and/or proton conducting membranes and devicesincorporating same, and applications for said membranes, particularly,but not limited to, fuel cells, gas sensors, and electrocatalyticdevices.

2. Description of Related Art

Ion conducting membranes are used in fuel cells, electrochemicalmembrane reactors, and in chemical sensors. In these exemplaryapplications, the membrane is electrically insulating but is conductiveto protons or oxygen ions. In fuel cells, the membrane performancelargely determines the fuel cell operating conditions and, as a result,the design of the entire fuel cell device. The two most common classesof fuel cells are polymeric electrolyte membrane fuel cells (PEMFCs) andsolid oxide fuel cells (SOFCs). The ion exchange membranes in PEMFCs arepolymers that function most effectively below 100° C. There are nopolymeric ion conducting membranes reported that operate effectivelyabove 200° C. The upper temperature limit on polymeric ion conductingmembranes means that expensive platinum catalysts should be used for theoxidation and reduction reactions in fuel cells.

The ion exchange membranes in SOFCs are ceramics that operate mosteffectively at temperatures above 700° C. The temperature is high enoughto allow non-precious metal catalysts to be effective for the oxidationand reduction reactions in fuel cells. However, the SOFC operatingtemperature is sufficiently high that stress from thermal cycles can,and often does, lead to device failure.

There is currently significant interest in developing effective andcommercially viable ion conducting membranes that can be used in anintermediate temperature range between about 200-600° C. The discoveryof an effective intermediate temperature ionic conducting membrane couldtruly revolutionize the fuel cell industry. The intermediate temperaturerange of 200-600° C. would be low enough to allow fuel cell constructionusing low cost materials, but high enough to use non-precious metalcatalysts and allow internal fuel reforming of hydrocarbon fuels.

Previous approaches to creating membranes suitable for the intermediatetemperature range have focused on either finding new ion conductors withhigher conductivity, or making existing membranes thinner to reduceoverall resistance.

Very thin ceramic membranes are too fragile to be self-supporting, andare typically supported in fuel cell devices by either the anode orcathode material. Ito et al., (“New Intermediate Temperature Fuel Cellwith Ultra-Thin Proton Conductor Electrolyte” J. Power Sources 2005,vol. 152, pp. 200-203) report a fuel cell that uses a palladium foilhydrogen membrane to support an ultrathin (˜700 nm thickness)BaCe_(0.8)Y_(0.2)O₃ ceramic known to exhibit purely protonicconductivity below 600° C. The palladium foil not only supports the thinproton conducting layer, but simultaneously serves as a fuel cell anodeand as a hydrogen membrane. After coating the proton conducting layerwith a perovskite ceramic cathode, they referred to the resulting threelayer structure as a “hydrogen membrane fuel cell” or HMFC, asillustrated generically in FIG. 1. At temperatures above 300° C.,hydrogen dissolves into the palladium in the form of protons andelectrons. The protons travel through the palladium foil and thenthrough the proton conducting ceramic. Since the proton conductingceramic is electrically insulating, the electrons are forced to travelfrom the palladium through an external circuit to the cathode, therebygenerating electricity. The reported performance of the HMFCdemonstrated a maximum power density of 0.9 W/cm² at 400° C. and 1.4W/cm² at 600° C.

One significant limitation in the HMFC described above is the use ofpulsed laser deposition to create the thin film of ceramic on palladium.Pulsed laser deposition is a high vacuum technique that is known to beunsuitable for economically coating large surface areas (the reportedHMFC was in the shape of a circle only six millimeters in diameter). Thepulsed laser deposition technique is also difficult if not impossible toemploy to coat non-planar substrates, such as the interior of a tube,for example.

Zeolite and molecular sieves have been reported in which mass transportoccurs through pores in the crystalline framework of the material. Thinzeolite and molecular sieve membranes can be deposited through chemicalmethods on both planar and tubular supports. Mass transport insidezeolite and molecular sieve crystals is typically anisotropic, with themost favorable mass transport occurring along one crystal axis.Multicrystalline zeolite and molecular sieve membranes with randomlyoriented crystal domains are not optimal for mass transport due to therandom mass transport path through the membrane and resistance to masstransport at boundaries between crystal domains. Lai et al.,(“Microstructural Optimization of a Zeolite Membrane for Organic VaporSeparation” Science 2003, Vol. 300 pp. 456-460 demonstrate enhancedzeolite membrane performance by adjusting chemical synthesis conditionsto optimize the membrane microstructure to promote mass transport. Theoptimized membranes have zeolite crystal domains aligned with thecrystal axis giving preferred mass transport oriented normal to themembrane surface. In addition, the crystal domains largely span themembrane thickness to reduce or eliminate resistive boundaries betweencrystal domains in the direction through the membrane thickness. Ananalogous microstructural optimization approach has not been extended toion or proton conducting membranes.

Hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂, or “HAP”) is a type of calciumphosphate that has a hexagonal crystallographic structure, which isthermally stable up to 1400° C. The stoichiometric Ca/P molar ratio is1.67 for stoichiometric HAP, but the apatite crystal structure can beformed with nonstoichiometric Ca/P ratios and with partial substitutionof other ions such as chlorine, fluorine, and yttrium into the crystalframework. High temperature electrochemical investigations haveindicated that HAP is proton conductive, with the mechanism ofconduction hypothesized to be migration of protons along hydroxyl groupslining the c-axis of the crystals. Since proton conduction occursprimarily along one crystal axis (c-axis) in HAP, it is expected thatconductivity will be strongly anisotropic in a single crystal. However,there is no reported study to date of high temperature protonconductivity in singe crystals of HAP due to the difficulty ofsynthesizing large-sized HAP single crystals.

Ban et al., “Hydrothermal-Electrochemical Deposition of Hydroxyapatite”,J. Biomed. Mater. Res., 1998, Vol. 42, pp. 387-395 and Ban et al.“Morphological Regulation and Crystal Growth ofHydrothermal-Electrochemically Deposited Apatite”, Biomaterials, 2002,Vol. 23, pp. 2965-2972 have reported electrochemical/hydrothermalsynthesis of thin films of hydroxyapatite on titanium and stainlesssteel electrodes to make the metal surfaces biocompatible for orthopedicimplants. Similar synthesis of hydroxyapatite crystals ontopalladium-based hydrogen membranes, useful for fuel cell applications,has not been reported. Electrochemical growth onto palladium membranesis particularly challenging due to hydrogen embrittlement. Embrittlementrefers to the membrane warping and damage that occurs when purepalladium is exposed to hydrogen at temperatures below 293° C. The useof palladium alloys rather than pure palladium mitigates warping to someextent, but does not eliminate issues of hydrogen embrittlement. Duringhydrothermal-electrochemical synthesis, hydroxyapatite nucleation andgrowth is driven by a local increase in pH near the cathode thataccompanies electrolysis of water. As a result, hydroxyapatite growsonly on the cathode, not the anode. Since hydrogen gas is evolved at thecathode during electrolysis, the hydroxyapatite cannot beelectrochemically deposited without exposing the palladium membranedirectly to hydrogen gas.

In view of the foregoing discussion and the known shortcomings ofcurrent technology, the inventors have recognized that improvements tothe current state of the art and solutions to the known problems in theart will be beneficial and advantageous. These improvements andsolutions will be set forth in the following description of embodimentsof the invention, the figures, and as recited in the appended claims.

SUMMARY DISCUSSION

An embodiment of the invention is directed to an ion-/proton-conductingmembrane. The membrane has selectively oriented crystal c-axes thatfacilitate (and are intended to optimize) ion/proton transport. Comparedto current ceramic membranes, the membrane is relatively thin and hassingle crystal domains spanning the membrane thickness. The membrane isalso sufficiently dense to provide a gas-tight barrier. By optimizingproton transport, a fuel cell incorporating such a membrane shouldoperate at lower temperatures than is currently possible with standardceramic membranes. In a non-limiting aspect, the film is apatitecrystals. In a more particular aspect, HAP is the ion-/proton-conductingmaterial. Alternative crystalline ion-/proton-conducting materials mayinclude, but are not limited to, zirconia, yttrium stabilized zirconia,lanthanum gallates, cerium dioxide, bismuth oxides, lanthanum-molybdenumoxides, brownmillerite, perovskite aluminates, apatite-type silicates,fluorite-type oxides, barium cerates, barium titanates, and strontiumcerates. The ion/proton conducting membrane further comprises asubstrate. According to a non-limiting aspect, the substrate ispalladium. Alternatively, the substrate may be a palladium alloy ornickel, or a non-metallic (e.g., ceramic) material. In an aspect, thesubstrate may be removable; i.e., a sacrificial substrate.

An embodiment of the invention is directed to a hydrogen fuel cell. Ifthe hydroxyapatite film on palladium (or nickel, for example) is coatedwith an electrically conducting cathode layer, the resulting structurewill be a hydrogen membrane fuel cell. The fuel cell includes an anode,a cathode, and an ion-/proton-conducting membrane disposed between theanode and the cathode, wherein the ion-/proton-conducting membranefurther comprises a hydroxyapatite (HAP) thin film having a thickness t.The HAP thin film is characterized by a plurality of single HAP crystalseach having its c-axis oriented normal to the substrate in the form of agas tight film, further wherein each of the single HAP crystals has acrystal domain that substantially spans the film thickness t. Accordingto an exemplary aspect, the cathode material isBa_(0.5)Sr_(0.5)Co_(0.8)Fe_(0.2)O_(3-δ (BSCF), a ceramic with very good performance as a fuel cell cathode in the temperature range of)500-700° C. Synthesized BSCF powder can be converted into a paste thatcan be painted onto, or otherwise applied to, an ion conducting ceramicand sintered to form a fuel cell cathode layer. Possible alternativeintermediate temperature cathode materials include other perovskiteceramics such as LaSrCoFeO, lithiated nickel oxides such as Li_(x)NiO₂₊(x=0.05-1.0), or other metal oxides with or without lithiation. The fuelcell will exhibit a proton conductivity equal to or greater than 1E-6S/cm over a temperature range between about 300° C. to 900° C. and, inan aspect will operate in the temperature range of 300° C.-600° C. Anexemplary device will be a new type of fuel cell membrane based on ionconduction through single crystals aligned to maximize proton transport.

An embodiment of the invention is directed to a method for making anion-/proton conducting membrane. The method includes the steps ofcreating a seeded surface by depositing a sufficiently dense HAP seedlayer onto a hydrogen membrane, hydrothermally synthesizing acrystalline HAP film on the seeded surface in a secondary, singlecrystallization that results in the crystal c-axis orientedsubstantially normal to the seeded surface, and hydrothermallydensifying the HAP to form a gas-tight thin film by a tertiarycrystallization that promotes a-axis crystal growth perpendicular to thec-axis. In a non-limiting aspect, the seed layer is directly grown on apure palladium hydrogen membrane acting as a cathode duringelectrochemical synthesis. By optimizing proton transport, fuel cellsmay have improved performance and be operated at lower temperatures thanpossible with standard ceramic membranes. In addition, by growing theproton conducting membrane electrochemically directly onto hydrogenmembranes, the proton conductor can easily and inexpensively be coatedonto large surface areas and onto tubular geometry often used forhydrogen membranes. According to an aspect of the method, palladiumembrittlement can be mitigated by reducing an amount of evolved hydrogenat the cathode electrode. This may be accomplished by applying anelectrical current equal to or less than 10 mA/cm² and/or limiting thedeposition time to between about five minutes to one minute. Accordingto an aspect, the method involves controlling supersaturation of HAP tomediate crystal nucleation and growth to achieve thin film densificationand/or to control the resulting thickness of the thin film;advantageously, controllably reducing the film thickness from about 10μm to 1-2 μm or less. According to another aspect, a crystal growthmodifier is added to alter growth rate of the crystal a-axis relative tothe crystal c-axis in order to promote growth of a dense gas-tight film.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows a generic hydrogen membrane fuel cell asknown in the art;

FIG. 2 schematically illustrates (a) the typical shape of a HAP singlecrystal, (b) the atomic environment around OH⁻ ions, and (c) protontransportation along the c-axis of the HAP crystal;

FIG. 3( a) schematically illustrates a randomly oriented HAP crystalmembrane structure; FIG. 3( b) schematically illustrates an ideal HAPmembrane structure with the c-axes of crystal domains spanning theentire membrane thickness to optimize proton transport;

FIG. 4 schematically shows (a) Seeding: an electrochemical seeddeposition of HAP on a Pd substrate; (b) Secondary growth: hydrothermaldeposition under conditions that favor c-axis growth to yield orientedcolumnar crystals; (c) Tertiary growth: hydrothermal deposition underconditions that favor a-plane growth to obtain oriented, dense(gas-tight) crystalline films, according to an illustrative embodimentof the invention;

FIG. 5 is a side-view SEM image showing the morphology of HAP depositedonto a 50 μm thick palladium/silver (75/25) alloy, according to anillustrative embodiment of the invention;

FIG. 6 a is a top-view SEM image; FIG. 6 b is a side-view SEM image,showing HAP seeds grown on Pd membrane, according to an illustrativeembodiment of the invention;

FIG. 7 a is a top-view SEM image of secondary growth of HAP crystals ona Pd membrane; FIG. 7 b is a side-view SEM image of FIG. 7 a, accordingto an illustrative embodiment of the invention;

FIG. 8 a is a top-view SEM image of tertiary growth of HAP crystals on aPd membrane; FIG. 8 b is a side-view SEM image of FIG. 8 a, according toan illustrative embodiment of the invention;

FIG. 9 schematically illustrates how surfactants assist HAP growth alongpreferential c- or a-axis directions, according to an illustrativeembodiment of the invention;

FIGS. 10( a-d) are X-ray diffraction (XRD) patterns of HAP seed and filmlayers in various growth processes, according to an illustrativeembodiment of the invention;

FIG. 11 shows an EDX spectrum of HAP film on a palladium substrate aftertertiary hydrothermal growth, according to an illustrative aspect of theinvention;

FIG. 12 shows an FTIR spectrum of HAP film on a palladium substrateafter tertiary hydrothermal growth, according to an illustrative aspectof the invention;

FIGS. 13( a-d) show SEM images of HAP seed layers on a palladiumsubstrate prepared by electrochemical deposition at 9.5 mA/cm² currentdensity in (a) 2 minutes and (b) 1 minute; (c) and (d) show themorphology of HAP films after tertiary hydrothermal synthesis grown fromthe seed layer (a) and (b), respectively, according to an illustrativeaspect of the invention;

FIGS. 14( a, b) show top and bottom SEM images, respectively, of thesurfaces of the HAP thin film shown in FIG. 13 d, according to anillustrative aspect of the invention;

FIG. 15 graphically shows measured proton conductivity (σ) of an HAPfilm, according to an illustrative aspect of the invention;

FIGS. 16 a, b show a SEM top-view image and a side-view image,respectively, of a Yttrium-substituted HAP film produced by seededhydrothermal growth according to an embodiment of the invention;

FIGS. 17 a, b show a SEM top-view image and a side-view image,respectively, of a Fluorine-substituted HAP film produced by seededhydrothermal growth according to an embodiment of the invention;

FIG. 18 graphically shows comparative fuel cell performance underdifferent temperatures measured on HAP films by seeded hydrothermalgrowth, according to an illustrative aspect of the invention; and

FIG. 19 graphically shows comparative fuel cell performance at 600° C.measured on HAP pellet and films prepared by different approaches(limiting current density from low to high: HAP sintered pellet (at1000° C. for 10 hours); HAP films produced by electrochemicaldeposition; HAP films produced by electrochemical deposition with waterglass coated gaps between crystals; HAP films produced byelectrochemical deposition with HAP gel-coated gaps between crystals;HAP films by seeded hydrothermal growth.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS OF THE INVENTION

As mentioned above, optimal performance of a hydroxyapatiteion-/proton-conducting membrane occurs when the crystal domains span theentire thickness of the membrane to eliminate grain boundary resistanceacross the thickness of the membrane. In addition, the crystal's c-axiswould be aligned so that the proton transport path is optimized, asillustrated in FIGS. 2( a-c).

FIG. 3( a) schematically shows HAP crystals that are randomly oriented.FIG. 3( b), on the other hand, schematically shows an ideal HAP membranestructure with the c-axes of crystal domains spanning the entiremembrane thickness to optimize proton transport;

A non-limiting, exemplary ion/proton conducting membrane 400-1 asillustrated in FIG. 4( c) includes a substrate 402 and a crystallineion-conducting thin film 304 (also shown in FIG. 3( b)) having athickness t. The thin film is characterized by a plurality of singleapatite crystals 304′ each having its c-axis oriented normal to thesubstrate (as also shown in FIG. 8( b) and FIGS. 13( a-c)). The apatitefilm is grown sufficiently dense (as described in more detail below) toform a gas-tight film (see, e.g., FIGS. 8( a, b). Each of the singlecrystals has a crystal domain that substantially spans the filmthickness t as illustrated in FIG. 3( b).

According to an aspect, the hydroxyapatite membrane is grown onto apalladium substrate. Two types of palladium substrates can beconsidered: thin foils and palladium deposited on porous supports.Palladium foils are currently commercially available in thicknesses aslow as 25 μm. Palladium foils have smooth surfaces that make themattractive as model substrates for viewing hydroxyapatite crystal growthvia electron microscopy. However, unsupported palladium foils areflexible. Bending of the palladium foil can cause the depositedhydroxyapatite layer to crack. For hydrogen membranes, palladium isoften deposited onto porous supports comprised of stainless steel,nickel, ceramics, glass, or silicon. The porous support providesmechanical stability, allowing very thin palladium membranes to be usedfor gas separation. Thinner membranes save cost by reducing the amountof palladium required per unit surface area and improve mass transportof hydrogen by lowering membrane resistance. Porous stainless steelplates and tubes can advantageously be used as palladium supports. Thepalladium is deposited following well-established electroless depositionprocesses after sensitizing the surface of the porous stainless steel inacidic PdCl₂ and SnCl₂ solutions.

Alternatively, the substrate may be a palladium alloy. FIG. 5 shows themorphology of HAP deposited onto a 50 μm thick palladium/silver 75/25alloy foil. It can be seen that the c-axis crystal growth is normal tothe substrate surface. Palladium/copper may also provide a suitablealloy.

In order to realize the ideal membrane structure illustrated in FIG. 4(c), “secondary growth” and “tertiary growth” processes are used, asillustrated in FIGS. 4( b), (c), respectively.

In both processes, a seed layer is first deposited to promote crystalgrowth on the surface as illustrated in FIG. 4( a). In the secondarygrowth process (FIG. 4( b)), a dense crystalline film is produced in asingle secondary crystallization onto the seeded surface. In a tertiarygrowth process, the secondary crystallization step promotes c-axisgrowth normal to the substrate but does not necessarily result in adense film. A tertiary crystallization step (FIG. 4( c)) is then appliedto promote a-axis growth (the a-axis is perpendicular to the c-axis) todensify the film and form a gas-tight barrier. To develop an effectivesecondary and tertiary growth processes for hydroxyapatite, a seed layermust first be formed with good surface coverage. Methods are then usedto control the crystallization kinetics, and adjust the growth rate ofthe crystal c-axis relative to the a-axis to grow a dense crystallinefilm onto the seed layer.

The following detailed description of seeding, secondary, and tertiarygrowth processes are exemplary, illustrative, demonstrative, andnon-limiting.

Preparation of HAP Seeds on Palladium Substrates

Palladium substrates were prepared by electroless plating of a thinlayer of palladium onto porous stainless steel supports following areported procedure. The porous stainless steel supports were cleanedwith an alkaline solution in an ultrasonic bath at ˜60° C. followed by asurface activation by successively immersing the supports in acidicbaths of SnCl₂ and PdCl₂. The palladium deposition was then conducted byimmersing the activated supports in a plating solution for approximatelythree hours. The plating procedure was repeated until a desired filmthickness was obtained. Typically, the thickness of the plated palladiumfilm was around 20 μm, which was thick enough to seal the pores at thesurface of the porous stainless steel supports. The porous stainlesssteel support ensures the mechanical stability of the thin palladiummembranes. Thin palladium film improves mass transport of hydrogen bylowering membrane resistance.

HAP seeds were deposited onto the palladium substrate surface via anelectrochemical deposition process. First, the palladium substrate waswashed with an industrial soap solution, ultrasonicated inethanol/acetone (volume ratio=50:50) solvent for 30 min, and then rinsedwith deionized water for use as a cathode electrode in theelectrochemical deposition reaction. The anode electrode was a platinumplate (25 mm×25 mm×127 μm). The anode and cathode electrodes were fixedonto two pieces of Teflon® material plates and arranged face to facewith a separation distance of 10 mm. The entire assembly was immersed inthe electrolyte solution preheated in an oil-bath (˜95° C.) for the HAPfilm growth. The electrolyte solution was prepared as described in theliterature, consisting of 1.67 mM K₂HPO₄, 2.5 mM CaCl₂ and 138 mM NaClin deionized water. The solution was buffered to pH 7.2 usingtris(hydroxymethyl)-aminomethane and 37% hydrochloride acid. A constantcurrent was generated with a direct current power supply and applied tothe electrodes for a desired time. After the electrochemical deposition,the cathode palladium electrode seeded with HAP crystals was taken outof the electrolyte solution, rinsed with deionized water several times,and dried in air.

Secondary Growth of HAP

The palladium substrate covered with the HAP seed layer was placed in aTeflon-lined vessel (40 ml of internal volume) with the seed layerfacing down and tilted up at approximately 45 degrees relative to thebottom of the vessel. The synthetic solution was prepared by dissolvingNa₂EDTA (0.20 M) and Ca(NO₃)₂ (0.20 M) in 15 ml deionized water, and(NH₄)₂HPO₄ (0.12 M) in the other 15 ml deionized water to form thecalcium and phosphate source solutions. The two source solutions weremixed together after pH of each solution was raised to 10.0 withammonium hydroxide. The synthetic solution was stirred at roomtemperature for about 20 minutes and then transferred into theTeflon-lined vessel to immerse the seed layer seated on the palladiumsubstrate. The Teflon-lined vessel was sealed in a stainless steelautoclave and put into a convective oven for the hydrothermal synthesisfor 15 hours at 200° C. and autogenous pressure. After the reaction, theautoclave was cooled to room temperature in a fume hood. The sample wastaken out, rinsed with deionized water several times, and dried in air.

Tertiary Growth of HAP

The set-up for tertiary hydrothermal growth was the same as secondarygrowth except that the HAP-covered palladium substrate after secondarygrowth was positioned in the Teflon-lined vessel. The synthetic solutionwas prepared by dissolving Na₂EDTA (0.20 M), Ca(NO₃)₂ (0.20 M), andcetylpyridinium chloride (0.02 M) in 15 ml deionized water sequentiallyat 40° C. under stirring to form a viscous clear calcium source solutionwith pH adjusted to 8.0 by ammonium hydroxide. The phosphate sourcesolution with pH ˜8.0 was prepared by dissolving (NH₄)₂HPO₄ in a second15 ml container of deionized water. The two solutions were mixedtogether, forming the synthetic solution for the tertiary hydrothermaldeposition. The tertiary growth was carried out for 60 hours at 200° C.and autogenous pressure. To keep sufficient calcium and phosphate ionsfor HAP film growth during the reaction, the synthetic solution wasreplaced with fresh solution after every 15 hours until a dense HAPcrystalline film was obtained.

Product Characterization

The crystal structure of HAP was determined by X-ray powder diffraction(XRD) with Cu Kα radiation (λ=1.5418 {acute over (Å)}). The integrationtime was 2 hours and the step size was 0.02 degrees/3 seconds. Images ofparticle morphology and elemental analysis of the products were obtainedusing a scanning electron microscope equipped with an energy dispersivex-ray (EDX) spectrometer. The Fourier transform infrared (FTIR) spectrumwas recorded with a spectrophotometer in the range of 500-2000 cm⁻¹. Thesample was measured with 32 scans at an effective resolution of 2 cm⁻¹.Proton conductivity measurements of the membranes were carried out usingtwo-point probe alternating current impedance spectroscopy over afrequency range of 300 kHz to 0.1 Hz. The upper surface of the HAP filmwas sputter coated with ˜300 nm of palladium/gold (60/40) alloy aselectrode. The electrical platinum leads were attached to both sides ofthe membrane using platinum paint. The membrane was attached to the endof an alumina tube using ceramic adhesive with the HAP film facingoutward. The tube was placed in a tube furnace for temperature control.To avoid palladium embrittlement, nitrogen was fed to the inside of thetube as it was heated. The gas flow was switched to hydrogen when thetemperature reached 300° C. The membrane was heated stepwise with a ramprate of 2° C./minute, with the membrane maintained at constanttemperature for at least one hour prior to each conductivitymeasurement.

Results and Discussion

Seeding HAP on Palladium Membranes

Electrochemical deposition has been widely studied for coating HAP ontitanium and titanium alloy substrates in the area of bioactive surfacemodification for orthopedic implants. Under appropriate syntheticconditions, an applied electric current causes a local increase in pHnear the cathode with the formation of OH⁻ n the electrolyte solutiondue to formation of hydrogen gas by the reduction of H₂O. The increasedpH and accumulation of both Ca²⁺ and HPO₄ ²⁻ ions near the cathoderesult in the supersaturation of calcium phosphate salts in thesolution. As a result, HAP nucleation and growth is driven by thesupersaturation onto the cathode metal electrode with the c-axis of thecrystal preferentially oriented normal to the electrode surface. Areported, typical recipe of electrochemical/hydrothermal synthesis ofHAP films on titanium substrates uses constant current conditions of˜12.5 mA/cm² at temperatures of 100-200° C. for a period of ˜1 hour.

The electrochemical deposition of HAP onto a palladium substrate hasnever been reported and is particularly challenging due to hydrogenembrittlement; i.e., the membrane warping and damage that occurs whenpure palladium is exposed to hydrogen at temperatures below 293° C.Since hydrogen gas is evolved at the cathode during electrolysis and HAPgrows only on the cathode, the HAP can not be electrochemicallydeposited without exposing the palladium membrane directly to hydrogengas. Under typical synthetic growth conditions of HAP on a titaniumsubstrate, palladium-based membranes are destroyed by hydrogenembrittlement. To minimize hydrogen embrittlement, a reduction inhydrogen production was obtained by applying a smaller electricalcurrent (typically less than 10 mA/cm²) and/or a shorter deposition time(less than 5 minutes), allowing the growth of an acceptable seed layeron the palladium substrate.

FIGS. 6( a, b) show an HAP seed layer with a porous structure on theunderlying palladium substrate formed in 4 minutes at 95° C. with acurrent density of 9.5 mA/cm². The crystals in the seed layer typicallyhave platelet morphology with a length of about 1-2 microns and asubmicron width, as shown in FIG. 6( a). FIG. 6( b) indicates that thecrystals preferentially orient normal to the palladium substrate. TheX-ray diffraction pattern in FIG. 10( b) identifies the crystals as HAP,formed most likely from the plate-like octacalcium phosphate, a crystalphase that typically appears at low pH and within a certain temperaturerange in the electrochemical deposition of HAP. The strongestdiffraction peak in FIG. 10( b) corresponds to (002) plane of thecrystals, which demonstrates a (002) (i.e., c-axis) preferredorientation normal to the palladium substrate. Some HAP crystals alsoorient along other directions as indicated by the presence of severalother diffraction peaks in FIG. 10( b). Typically, in a hexagonal HAPcrystal, the cross-sectional surface, which is hexagonal in shape, isthe c surface, which is parallel to the a-axis of the crystal unit cell.The six surfaces (rectangular in shape) occurring along the c-axis ofthe crystal unit cell are a surfaces (see FIG. 2 a). The HAP seed layerpromotes adhesion of the seed crystals to the underlying substrate andenhances secondary and tertiary growth of oriented HAP films at thesubstrate surface.

HAP Secondary Hydrothermal Growth

Hydrothermal synthesis with a mechanism of calcium chelate decompositionhas been studied to grow large sized HAP crystals. A chelating agent,usually a carboxylic acid such as acetic acid, lactic acid, citric acid,and ethylenediamine tetraacetic acid (EDTA), is used to bind calciumions to form a homogeneous phosphate-containing solution for thereaction. Upon heating the solution, the calcium carboxylate decomposesas the chemical equilibria shift and calcium is slowly and continuouslyreleased into the phosphate-containing solution. As the solution becomessupersaturated, calcium phosphate crystals progressively nucleate andgrow. In other words, the crystal nucleation and growth are mediatedwith a controlled supersaturation by calcium chelating decomposition.Without the chelating agent, the solution would be supersaturated andthe most stable calcium phosphate phase would spontaneously precipitateat the beginning of the reaction.

FIGS. 7( a, b) and FIG. 10( c) show the surface and cross-sectionalmorphology, and XRD pattern, respectively, of an HAP crystal layer grownon a seeded palladium substrate after a secondary hydrothermaldeposition in a calcium-Na₂EDTA chelating solution. In FIG. 7( a), mostcrystals are rod-like in shape, having a well-defined hexagonal crystalhabit with a width up to approximately 2 μm. The rod-shaped crystalsorient perpendicular all the way down to the substrate with a length ofapproximately 7 m, as shown in FIG. 7( b). The XRD pattern of the HAPfilm, illustrated in FIG. 10( c), shows a strong enhancement inintensity of (002) diffraction peak in comparison with shrinkage ordisappearance of other diffraction peaks. The enhanced (002) intensityindicates that HAP crystals are c-axis oriented normal to the palladiumsubstrate, consistent with the SEM observation in FIG. 7( b). Thehexagonal surface is the c-surface of the crystals, as visualized in thetop-view SEM image in FIG. 7( a). The six a-surfaces situating along thec-axis perpendicular to the c-surface of the crystals can be visualizedin the side-view SEM image in FIG. 7( b).

The morphology and orientation of HAP crystals developed in thesecondary hydrothermal growth are similar to those prepared by thehydrothermal-electrochemical deposition for a long deposition time ontitanium substrates. The damage of palladium membrane due to hydrogenembrittlement, as discussed in HAP seeding step, prevents growth oflarge HAP crystals with a purely electrochemical deposition process. Thehydrothermal synthesis with a calcium chelate decomposition controls HAPsupersaturation and promotes HAP growth with aligned c-axes on theseeded substrate surface rather than in the phosphate-containingsolution. The seeded surface is important for the further growth of HAPinto a denser crystalline film. A control experiment using unseededpalladium substrate was performed to investigate the effect of the seedlayer. After the reaction, no uniform deposition was achieved, and onlya few crystal aggregates consisting of rod-like HAP crystals aredeposited as widely separated islands on the surface of the substrate.

HAP Tertiary Hydrothermal Growth

Secondary hydrothermal growth of the seed layer (FIG. 7) yieldeddesired, highly c-axis-oriented rod-like crystals, but did not result ina dense HAP film. A tertiary growth step was used to promote lateralintergrowth of crystals to achieve a gas-tight HAP thin film. Synthesisof HAP has shown that HAP tends to form elongated whiskers along thec-axis of the crystal with well developed a-surfaces. Accelerating HAPgrowth along the a-axis direction (i.e., perpendicular to the c-axis)with developed c-surfaces results in crystals forming a dense, gas-tightthin film. Studies of HAP as a liquid chromatography packing indicatethat HAP crystals have positively-charged a-surfaces andnegatively-charged c-surfaces. The addition of oppositely-chargedadditives such as surfactants may result in electrostatic adsorption ofmolecules onto specific a- or c-surfaces to slow down or inhibit thecrystal growth along one specific direction by mediating the crystalgrowth kinetics. To alter HAP growth preferentially along the a-axisdirection, the cationic surfactant cetylpyridinium chloride was added ina tertiary hydrothermal growth of the HAP film.

FIGS. 8( a, b), and FIG. 10( d) show the morphology and XRD pattern,respectively, of an HAP film grown on the crystal layer by a tertiaryhydrothermal growth. The HAP film appears as a dense and stronglyadherent layer on the substrate. FIG. 8( a) is a top-view of the filmthat shows the crystal domains grown together over several microns,although some small gaps remain. The small gaps do not extend completelythroughout (i.e., to the bottom of) the film, as visualized in theside-view SEM image in FIG. 8( b). The crystal film is oriented normalto the palladium substrate with a thickness of approximately 25 μm. Thecrystals in the film appear larger both in width and length than thoseafter secondary growth, which indicates that HAP crystallization occursalong not only the a-axis direction, but also the c-axis direction inthe tertiary deposition step. The rate of crystal growth in the a-axisdirection is increased, while decreased in the c-axis direction, to forma dense, gas-tight film in the tertiary step. The crystal c-axisorientation is further verified with XRD characterization, as shown inFIG. 10( d). The peak corresponds to the (002) reflection plane. Thisconfirms that during the tertiary treatment, HAP crystal growth followsthe orientation of the original plate-like seeds in the seeding step androd-like crystals in the secondary step, while the growth ofmis-oriented crystals is minimal or nonexistent in the tertiaryhydrothermal deposition.

The foregoing SEM and XRD characterizations indicate that HAP crystalsgrow together with c-axes normal to the underlying palladium substrate.Cationic cetylpyridinium chloride most likely functions to assistcrystal growth along a-axes. As schematically illustrated in FIG. 9, thepositively charged surfactant is preferentially adsorbed to thenegatively charged c-surfaces through electrostatic interactions. Theadsorbed surfactant limits diffusion of the calcium and phosphate ionsonto the c-surfaces to slow down growth along the c-axis direction. As aresult, sufficient calcium and phosphate ions diffuse to the a-surfacesto accelerate growth along the a-axis direction, and finally to growcrystals together. To further verify the effect of cetylpyridiniumchloride on the crystal growth, a control experiment to synthesize HAPfilm under the same conditions without addition of cetylpyridiniumchloride was conducted. The result shows that individual crystals in thefilm grew longer than 25 microns, but many gaps between crystals wereobserved to extend down to the bottom of the film.

The elemental composition and Ca/P ratio of the HAP films were studiedwith EDX spectroscopy, as shown in FIG. 11. The EDX spectrum iscomprised of 0, P and Ca peaks, confirming the presence of HAP. The Cpeak is likely from carbon tape used for the SEM observation. Asemi-quantitative analysis of the EDX spectrum shows that the Ca/Patomic ratio is around 1.53, a little lower than the idealstoichiometric value of 1.67. The nonstoichimetry (Ca deficiency) of HAPis consistent with reports from the literature for synthesis ofmicro-sized HAP crystals in hydrothermal reactions. The FTIR spectrum ofthe HAP film illustrated in FIG. 12 shows all absorption bandscharacteristic for HAP. The librational vibration of OH⁻ groups appearat 632 cm⁻¹, whereas the internal modes corresponding to the PO₄ ³⁻groups centered at a range of wavenumbers can be clearly determined fromthe spectrum. Absence of any distinct bands in the range of 1400-1500cm⁻¹ indicates that HAP does not contain large quantities of carbonateions. The band observed at 870 cm⁻¹ is ascribed to the HPO₄ ²⁻ groups,in agreement with data reported in the literature. The presence of HPO₄²⁻ groups is consistent with the low Ca/P ratio in the synthesized HAPcrystalline films.

HAP Films with Tunable Thickness

To make an effectively working fuel cell, a thin electrolyte membranewill advantageously reduce the membrane resistance. A typical thicknessof the HAP membranes after the seeded growth is about 25 μm. A fuel cellHAP membrane film thickness will advantageously be less than 25 μm.According to an embodiment of the invention, membrane thickness can bereduced by using a shorter deposition time in the seed layer formationand/or a higher molar ratio of calcium to phosphate (or, lower phosphateconcentration) in the hydrothermal depositions.

FIGS. 13( a-d) show SEM images of HAP seed layers on a palladiumsubstrate prepared by electrochemical deposition at 9.5 mA/cm² currentdensity in (a) 2 minutes and (b) 1 minute; (c) and (d) showing themorphology of HAP films after tertiary hydrothermal synthesis grown fromthe seed layer (a) and (b), respectively. As shown in FIGS. 13( a, b),the seed layer is about 300 nm and 600 nm thick when the electrochemicaldeposition time was 2 and 1 minutes, respectively. The secondary andtertiary hydrothermal synthesis using these two seeded surfaces occurredunder conditions similar to grow approximately 25 μm thick films, exceptthe phosphate concentration was reduced to 0.01 M from the originalphosphate concentration of 0.06 M. After the reaction, the synthesizedfilms were reduced to 5 and 2.5 μm thick, respectively, as shown in FIG.13( c, d). The reduced film thickness is partially due to the thin seedlayers produced in the electrochemical deposition. The reduced phosphateconcentration may lower the degree of supersaturation of HAP in thereaction, and as a result alter the crystal growth habit by slowing downthe crystal growth along the c-axis, and promoting the growth along thea-axis into a dense thin film. The HAP thin films still have a dense andcontinuous morphology on the surfaces of the approximately 2.5 μm thickfilms, as shown in FIG. 14. The top surface (FIG. 14 a) indicates thatthere are some crystal domains, but all of them are grown together intoa dense film. The bottom surface (FIG. 14 b) is smooth and dense. A fewsmall protrusions in the bottom surface result from the palladiumsubstrate because the electroless-plated substrate is not perfectly flatand smooth.

Proton Conductivity of HAP Films

FIG. 15 shows proton conductivity of an HAP thin film on a palladiumsubstrate prepared by the electrochemical and hydrothermal depositionprocesses described herein above. Electrical impedance spectroscopy wascarried out to characterize the proton conductivity of the films. Themeasurement was conducted on an HAP film approximately 25 μm thick,similar to that shown in FIG. 8, created after seeded hydrothermalgrowth. FIG. 15 shows the resulting proton conductivity as a function oftemperature up to 900° C. At 100-200° C., the measured conductivity wasvery low, approximately 10⁻⁹ S/cm. At 300° C., the conductivity wasapproximately 10⁻⁷ S/cm while nitrogen was being fed to the tube. Theconductivity jumped to approximately 10⁻⁵ S/cm at 300° C. afterswitching to hydrogen flow into the tube. The enhancement of protonconductivity at 300° C. when the membrane was exposed to a hydrogenatmosphere is due to the injection of protons from the palladiummembrane to the HAP thin film. The injected protons increase the numberof transporting protons in the crystalline membrane and, as a result,improve the membrane proton conductivity. The conductivity increasedsteadily with temperature above 300° C., reaching approximately 10⁻³S/cm at 800° C., due to the c-axis-aligned crystal domains (it should benoted that interfacial resistance between the electrodes and the HAP maygive a non-negligible contribution to the overall measured resistancebecause the film thickness is small (˜25 μm)). In comparison, atraditional sintered HAP ceramic in dry air at 800° C. has aconductivity of approximately 5×10⁻⁷ S/cm, nearly four orders ofmagnitude lower, suggesting that the HAP membrane with aligned crystaldomains according to an embodiment of the invention may be capable ofgiving improved fuel cell performance.

Conductivity may further be increased by a) yttrium and b) fluorinesubstitution.

Synthesis and SEM characterization of yttrium substituted hydroxyapatite(Y-HAP)

Preparation of Electrolyte

The electrolyte was prepared by adding 125 ml 50 mMtris(hydroxyl)aminomethane (Tris) (99.8+%, ACS reagent, Aldrich) into a250 ml beaker, followed by adding 1.006 g sodium chloride (NaCl) (99+%,ACS reagent, Aldrich), 0.046 g calcium chloride dihydrate (CaCl₂.2H₂O)(99+%, ACS reagent, Aldrich), and 0.037 g potassium hydrogen phosphate(K₂HPO₄) (99.99%, Aldrich)) in sequence. The solution changed from clearto opaque after K₂HPO₄ was introduced. The pH of the solution was 9.78.An adequate amount of HCl (38%, Mallinckrodt Chemicals) was used totitrate the solution to a pH of 7.20. The solution returned to clearafter the titration process.

Electrochemical Deposition of HAP

The beaker containing the electrolyte was transferred to a preheated oilbath. After approximately 1 hour, the electrolyte temperature wasstabilized at 95° C. A constant electric current was applied using a DCpower supply. The current density was set to 25.0 mA/cm² (based on thearea of the Pd cathode) for 4 min. A magnetically coupled stir barturning at 600 rpm was utilized to stir the bath throughout thedeposition process. After the electrochemical deposition ofhydroxyapatite, the cathode electrode was taken out of the electrolyte,rinsed with deionized water several times and dried in air. Theresulting HAP film on the cathode was used as the seed layer for thepost-growth (secondary and tertiary growth) of HAP in hydrothermalsynthesis.

Secondary Growth

The synthetic solution was prepared by dissolving Ca(NO₃)₂, Y(NO₃)₃ andNa₂EDTA in 15 ml deionized water, and (NH₄)₂HPO₄ was dissolved in theother 15 ml deionized water under a mild magnetic stirring. The twosolutions were then mixed together after the pH was raised to 10 withapproximately 28% ammonium hydroxide, respectively. The final aqueoussolution contained 0.10 M Ca(NO₃)₂, 0.02 M Y(NO₃)₃, 0.13 M Na₂-EDTA and0.06 M (NH₄)₂HPO₄. The solution was transferred into a 40 ml Teflonliner situated in a stainless steel autoclave. The HAP seeded electrodewas fixed onto a Teflon plate and placed inside the synthetic gel withthe seeded side facing down to the bottom of the Teflon liner. Theautoclave was closed tightly and moved into a preheated gravityconvection oven at 200° C. for 15 hours. After the reaction, theautoclave was cooled to room temperature in a fume hood. The sample wastaken out, rinsed with deionized water several times, and dried in air.

Scanning Electron Microscopy

Images of crystalline HAP films were taken with a scanning electronmicroscope at an accelerating voltage of 10 kV. FIGS. 16 a, b show atop-view image and a side-view image, respectively, of Y-HAP filmsproduced by seeded hydrothermal growth according to an embodiment of theinvention.

Yttrium nitrate can be added along with calcium nitrate duringhydrothermal synthesis to obtain crystalline films with varying levelsof yttrium substitution.

Synthesis and SEM characterization of fluorine substitutedhydroxyapatite (F-HAP)

Preparation of Electrolyte

This was the same as for the synthesis of yttrium substitutedhydroxyapatite, above.

Electrochemical Deposition of HAP

This was the same as for the synthesis of yttrium substitutedhydroxyapatite, above.

Secondary Growth

A synthetic solution was prepared by dissolving Ca(NO₃)₂ and Na₂EDTA in15 ml deionized water; and NH₄F and (NH₄)₂HPO₄ in the other 15 mldeionized water under a mild magnetic stirring. The two solutions werethen mixed together after pH was raised to 10 with approximately 28%ammonium hydroxide, respectively. The final aqueous solution contained0.10 M Ca(NO₃)₂, 0.10 M Na₂-EDTA, 0.01 M NH₄F and 0.01 M (NH₄)₂HPO₄. Thesolution was transferred into a 40 ml Teflon liner, situated in astainless steel autoclave. The HAP seeded electrode was fixed onto aTeflon plate and placed inside the synthetic gel with the seeded sidefacing down to the bottom of the Teflon liner. The autoclave was closedtightly and moved into a preheated gravity convection oven at 200° C.for 15 hours. After the reaction, the autoclave was cooled to roomtemperature in a fume hood. The sample was taken out, rinsed withdeionized water several times, and dried in air.

Tertiary Growth

The synthetic solution was the same as that in secondary growth exceptthat cetylpyridium chloride (0.01 M) was added in the reactant solution.All experimental procedures were the same as those used for secondarygrowth.

Scanning Electron Microscopy

Images of crystalline HAP films were taken with a scanning electronmicroscope at an accelerating voltage of 10 kV. FIGS. 17 a, b show atop-view image and a side-view image, respectively, of F-HAP filmsproduced by seeded hydrothermal growth according to an embodiment of theinvention.

Comparative Fuel Cell Performance

FIG. 18 graphically shows comparative fuel cell performance underdifferent temperatures measured on HAP films by seeded hydrothermalgrowth, according to an illustrative aspect of the invention. Assemblyof the test-cell was completed as follows: the cathode catalyst wasprepared by sintering BSCF/PEG slurry at 950° C. for 1 hour to form aporous pellet. A platinum (Pt) paste was coated to the porous pellet bybrush-coating and sintering at 900° C. for 0.5 hour. The cathode BSCF/Ptpellet was attached to the HAP membrane and a Pt mesh was attached tothe cathode as a current collector. Current and potential leads weresimilarly attached to the anode (Pd foil) and cathode (Pt mesh) for theperformance measurement. The test-cell was then seated across the top ofan alumina tube using ceramic adhesive with the cathode facing outward.The alumina tube was placed in a tube furnace for temperature control. Asimilar procedure to the HAP conductivity measurement was used here tomeasure the fuel cell performance

FIG. 19 graphically shows comparative fuel cell performance at 600° C.measured on HAP pellet and films prepared by different approaches(limiting current density from low to high: HAP sintered pellet (at1000° C. for 10 hours); HAP films produced by electrochemicaldeposition; HAP films produced by electrochemical deposition with waterglass coated gaps between crystals; HAP films produced byelectrochemical deposition with HAP gel-coated gaps between crystals;HAP films by seeded hydrothermal growth.

In summary, c-axis oriented HAP thin films synthesized by seeded growthon a palladium hydrogen membrane substrate have been disclosed. Anexemplary synthetic process included electrochemical seeding on thesubstrate, and secondary and tertiary hydrothermal treatments underconditions that favor growth along c-axis and a-axis in sequence. Byadjusting corresponding synthetic conditions, an HAP this film can begrown to a controllable thickness with a dense coverage on theunderlying substrate. Proton conductivity measurement showed that thethin films have relatively high conductivity under hydrogen atmosphereand high temperature conditions. The c-axis oriented films obtained bythe embodied technique may be integrated into fuel cells for applicationin the intermediate temperature range of 200-600° C. Theelectrochemical-hydrothermal deposition technique disclosed herein maybe applied to create other oriented crystal materials having optimizedproperties, useful for separations and catalysis as well as electronicand electrochemical applications, electrochemical membrane reactors, andin chemical sensors.

Having thus described the various embodiments of the invention, it willbe apparent to those skilled in the art that the foregoing detaileddisclosure is presented by way of example only and thus is not limiting.Various alterations, improvements and modifications recognized by thoseskilled in the art, though not expressly stated herein, may be made andare intended to be within the spirit and scope of the claimed invention.

Additionally, the recited order of processing elements or sequences, orthe use of numbers, letters, or other designations, is not intended tolimit the claimed processes to any order except as may be specified inthe claims. Accordingly, embodiments of the invention are limited onlyby the following claims and equivalents thereto.

1. An ion/proton conducting membrane, comprising: a substrate; and acrystalline ion-conducting thin film having a thickness t, wherein thethin film is characterized by a plurality of single apatite crystalseach having its c-axis oriented normal to the substrate in the form of agas tight film, further wherein each of the single crystals has acrystal domain that substantially spans the film thickness t.
 2. Theion/proton conducting membrane of claim 1, wherein the membrane has aproton conductivity equal to or greater than 1E-6 Siemens per centimeterover a temperature range between about 300° C. to 900° C.
 3. Theion/proton conducting membrane of claim 1, wherein the substrate ismetallic.
 4. The ion/proton conducting membrane of claim 1, wherein thesubstrate is a removable, sacrificial material.
 5. The ion/protonconducting membrane of claim 3, wherein the substrate is palladium. 6.The ion/proton conducting membrane of claim 3, wherein the substrate isnickel.
 7. The ion/proton conducting membrane of claim 5, wherein thepalladium substrate is an unsupported thin film.
 8. The ion/protonconducting membrane of claim 5, wherein the palladium substrate includesa porous support.
 9. The ion/proton conducting membrane of claim 8,wherein the porous support is stainless steel.
 10. The ion/protonconducting membrane of claim 7, wherein the palladium substrate has athickness of about 20 microns to 30 microns.
 11. The ion/protonconducting membrane of claim 3, wherein the substrate is palladiumalloyed with another metal.
 12. The ion/proton conducting membrane ofclaim 11, wherein the another metal includes at least one of silver andcopper.
 13. The ion/proton conducting membrane of claim 1, wherein thecrystalline ion-conducting thin film is hydroxyapatite (HAP).
 14. Theion/proton conducting membrane of claim 13, wherein the film thicknessis in a range between about 2 micron to 30 micron.
 15. The ion/protonconducting membrane of claim 13, wherein the film thickness is in arange between greater than zero to 10 micron.
 16. The ion/protonconducting membrane of claim 15, wherein the film thickness, t, is in arange 5 micron≦t≦2 microns.
 17. The ion/proton conducting membrane ofclaim 13, wherein the single HAP crystals are substantially rod-shaped.18. The ion/proton conducting membrane of claim 13, wherein the HAP thinfilm is substantially free of grain boundaries across the thickness t.19. The ion/proton conducting membrane of claim 1, wherein the thin filmis a hydrothermally-grown, electrochemically-seeded structure that isdirectly adhered to the substrate.
 20. The ion/proton conductingmembrane of claim 1, wherein the thin film is yttrium-substituted HAP.21. The ion/proton conducting membrane of claim 1, wherein the thin filmis fluorine-substituted HAP.
 22. The ion/proton conducting membrane ofclaim 13, wherein the HAP thin film further comprises a sol-gel coatingof HAP.
 23. An ion/proton conducting membrane, comprising: a substrate;and a hydroxyapatite (HAP) thin film having a thickness t, wherein theHAP thin film is characterized by a plurality of single HAP crystalseach having its c-axis oriented normal to the substrate in the form of agas tight film, further wherein each of the single HAP crystals has acrystal domain that substantially spans the film thickness t.
 24. Theion/proton conducting membrane of claim 23, wherein the substrate ispalladium.
 25. The ion/proton conducting membrane of claim 23, whereinthe substrate is nickel.
 26. The ion/proton conducting membrane of claim23, wherein the substrate is a palladium alloy.
 27. The ion/protonconducting membrane of claim 26, wherein the alloy is one ofpalladium/silver and palladium/copper.
 28. The ion/proton conductingmembrane of claim 23, wherein the film thickness is in a range betweengreater than zero to 10 microns.
 29. The ion/proton conducting membraneof claim 28, wherein the film thickness is in a range between about 2microns to 5 microns.
 30. The ion/proton conducting membrane of claim23, wherein the single HAP crystals are substantially rod-shaped. 31.The ion/proton conducting membrane of claim 23, wherein the HAP thinfilm is substantially free of grain boundaries across the thickness t.32. The ion/proton conducting membrane of claim 23, wherein the thinfilm is a hydrothermally-grown, seeded structure that is directlyadhered to the substrate.
 33. The ion/proton conducting membrane ofclaim 23, further comprising a cathode material layer on the top surfaceof the thin film.
 34. The ion/proton conducting membrane of claim 33,wherein the cathode material layer isBa_(0.5)Sr_(0.5)CO_(0.8)Fe_(0.2)O_(3-δ) (BSCF).
 35. The ion/protonconducting membrane of claim 33, wherein the cathode material layer isLaSrCoFeO.
 36. The ion/proton conducting membrane of claim 33, whereinthe cathode material layer is a metal oxide.
 37. The ion/protonconducting membrane of claim 36, wherein the metal oxide is a lithiatednickel oxide (Li_(x)NiO₂₊ (x=0.05-1.0)).
 38. A hydrogen fuel cell,comprising: an anode; a cathode; and an ion-/proton-conducting membranedisposed intermediate the anode and the cathode, wherein theion-/proton-conducting membrane further comprises: a palladium hydrogensubstrate; and a hydroxyapatite (HAP) thin film having a thickness t,wherein the HAP thin film is characterized by a plurality of single HAPcrystals each having its c-axis oriented normal to the substrate in theform of a gas-tight film, further wherein each of the single HAPcrystals has a crystal domain that substantially spans the filmthickness t.
 39. The hydrogen fuel cell of claim 38, characterized by aproton conductivity equal to or greater than 1E-6 Siemens per centimeterover a temperature range between about 300° C. to 900° C.
 40. A methodfor making an ion-/proton conducting membrane, comprising: creating aseeded surface by depositing a sufficiently dense HAP seed layer on ahydrogen membrane cathode electrode; hydrothermally synthesizing ac-axis-oriented, crystalline HAP film on the seeded surface in asecondary, single crystallization substantially normal to the seededsurface; and hydrothermally densifying the HAP film in an a-axisorientation perpendicular to the c-axis orientation in a tertiarycrystallization to form a gas tight thin film.
 41. The method of claim40, comprising depositing the seed layer on the hydrogen membranecathode electrode by one of a spin-coating and dip-coating HAPparticles.
 42. The method of claim 41, comprising depositing the seedlayer on a non-metallic hydrogen membrane cathode electrode.
 43. Themethod of claim 40, comprising electrochemically depositing the seedlayer on the hydrogen membrane cathode electrode.
 44. The method ofclaim 43, comprising electrochemically depositing the seed layer on apure palladium hydrogen membrane cathode electrode.
 45. The method ofclaim 44, comprising reducing an amount of evolved hydrogen at thecathode electrode by at least one of applying an electrical currentequal to or less than 10 milliAmperes per squared centimeter (mA/cm²)and limiting the deposition time to between about five minutes to oneminute.
 46. The method of claim 40, wherein during the secondarycrystallization, controlling supersaturation of HAP to mediate crystalnucleation and growth with one of a calcium ion chelator and a pHregulator.
 47. The method of claim 40, wherein the calcium ion chelatorcomprises one of EDTA and bis(2-ethylhexyl)sulfosuccinate.
 48. Themethod of claim 40, wherein the pH regulator comprises one of urea andhexamethylenetetramine.
 49. The method of claim 40, wherein during thetertiary crystallization, controlling the c-axis/a-axis growth by addinga cationic surfactant during the hydrothermal growth of the HAP film.50. The method of claim 49, comprising adding cetylpyridinium chloride.51. The method of claim 40, wherein during the tertiary crystallization,controlling the c-axis/a-axis growth by varying the ratio of calcium tophosphorous.
 52. The method of claim 40, wherein during the tertiarycrystallization, controlling the c-axis/a-axis growth with a crystalgrowth modifier selected from the group consisting of:tetramethylammonium hydroxide, tetraethylammonium hydroxide,tetrabutylammonium hydroxide, tetraethylammonium bromide,tetrapropylammonium bromide, tetrabutylammonium bromide,tetrahexylammonium bromide, tetraoctylammonium bromide, triethanolamine,trimethyl(tetradecyl)ammonium bromide, benzyltriethylammonium chloride,benzyldimethyldodecylammonium chloride, poly(diallydimethylammoniumchloride), 1-ethyl-2-methylquinolinium iodide, and cetyltrimethylammonium bromide.
 53. The method of claim 40, comprising controlling thethickness of the HAP thin film to a thickness of about 10 microns orless by at least one of limiting the electrochemical deposition time tobetween about five minutes to one minute and creating a higher molarratio of calcium to phosphate during at least one of the hydrothermaldepositions.
 54. The method of claim 40, comprising adding a crystalgrowth modifier to alter the growth rate of the crystal a-axis relativeto the crystal c-axis.